1. Technical Field
The present invention relates to an artificial spinal disc or prosthesis to replace a damaged or degenerated spinal disc.
2. Description of the Prior Art
An artificial spinal disc desirably should be capable of acting as a natural disc. The artificial disc should maintain the vertebrae spaced from each other and prevent pinching of nerves or spinal cord. The artificial disc should carry load and transmit load between the vertebrae adjacent the disc with an even distribution of the load across the disc. Further, the artificial disc should be sufficiently resilient to enable relative turning (flexion) of the vertebrae adjacent the disc (as upon turning of the shoulders of the patient having the disc). Research has shown that a natural spinal disc enables angular flexion between 2.degree. and 3.degree.. The artificial disc must also provide resistance to turning as does a natural disc so that excessive turning of one vertebra relative to the other is not possible. Also, the artificial disc should be resilient to accommodate all other motions of the spine including flexion, extension, lateral bending as well as combinations of these motions. Again, excessive motion of one vertebra relative to another should not be possible. The artificial disc should be both biocompatible and biostable such that the disc itself or any of its degradation by products, if any exist, do not cause adverse tissue reactions.
U.S. Pat. No. 3,867,728 discloses a prosthetic spinal disc which in one form comprises a reinforced resilient block of elastomer, such as silicone rubber or polyurethane. The elastomer is reinforced by fibrous material such as Dacron filaments embedded in the silicone elastomer. The upper and lower surfaces of the disc can be open-pore, tissue-ingrowth receptive surfaces.
Prior U.S. Pat. No. 4,309,777 discloses an artificial spinal disc comprising upper and lower disc portions of a metal such as stainless steel. The disc portions are held in a spaced-apart relationship by a plurality of compression springs. A plurality of spikes extend upwardly and downwardly from the exposed surfaces of the disc portions. The springs can be varied in size and number to vary the size of the disc had to achieve a desired vertebra separation.
Prior U.S. Pat. No. 4,714,469 discloses a spinal implant comprising a rigid solid body having opposed upper and lower surfaces, elongated protuberances of substantially semi-circular cross section extending the full width of the upper and lower surfaces, and porous coatings covering said protuberances. The coatings comprise two layers of substantially spherical particles of the same alloy as the disc body. The coatings can also cover the upper and lower surfaces of the body, in addition to the surfaces of the protuberances. The coatings provide for tissue/bone ingrowth. The disc of this patent does not offer the flexibility of a human disc, either with regard to relative rotation of adjacent vertebrae or with regard to relative axial movement of the vertebrae.
Prior U.S. Pat. No. 4,759,766 discloses an artificial disc comprising first and second end plates and a piece of hard plastic such as polyethylene or polyurethane of high compression and tension strength interposed between the end plates. The end plates can have a number of configurations. The end plates may be provided with teeth to guarantee an anchorage in the opposed vertebrae.
Prior U.S. Pat. No. 4,743,256, in FIG. 12, discloses a plug dimensioned and shaped to fit and maintain the disc space between adjacent vertebrae. Bone piercing tangs penetrate the vertebrae. The plug is preferably made of an inert metal substrate having a porous metal coating thereon.
Prior publication entitled "Characterization of Hexsyn, a Polyolefin Rubber", by McMillian, Journal of Biomaterials Applications, Vol. 2, July, 1987, pages 3-100, discloses a polyolefin rubber for use in biomedical applications. The rubber is biocompatible and fatigue resistant. It is synthesized from 1-hexane with 3-5% methylhexadiene as the source of residual double bonds for vulcanization. A primary use for the rubber components is in ventricular assist and artificial heart systems. This rubber is used as the hinge portion of prostheses, such as finger joints. The Journal article gives a number of physical properties of the material such as tensile strength, elongation and elastic modulus.